Generally, vinyl chloride-based resins are often formed into molded articles through a step of adding a plasticizer to a vinyl chloride-based resin and molding the resulting vinyl chloride-based resin composition.
For these molded articles (molded products) obtained by molding a vinyl chloride-based resin composition containing a plasticizer, various types of performance have been in demand, such as flexibility, cold resistance, heat resistance, and electrical properties. Examples of typical plasticizers for vinyl chloride-based resin include phthalate ester plasticizers for vinyl chlorides, such as di-2-ethylhexyl phthalate (hereinafter referred to as “DOP”) and diisononyl phthalate (hereinafter referred to as “DINP”). These plasticizers have been commonly used.
However, in recent, years, safety of chemical substances has been drawing attention, and since toxicity of phthalate ester was confirmed when a large amount of phthalate ester was administered to rodents, phthalate ester use is now restricted mainly for infants in Japan and Western countries as a preventive measure. In the field of plasticizers, a non-phthalate ester plasticizer, more specifically, a plasticizer free of esters of phthalic acid and alcohol having 8 or fewer carbon atoms, or a plasticizer containing a small amount of such esters, has been desired in the market. For example, in Europe, the content of phthalate ester in molded resin articles is strictly limited (REACH (Regulation (EC) No 1907/2006)), and even a very small amount of phthalate ester component in a molded article is becoming problematic.
To date, acetyltributyl citrate (hereinafter referred to as “ATBC”), di-2-ethylhexyl adipate (hereinafter referred to as “DOA”), tri-2-ethylhexyl trimellitate (hereinafter referred to as “TOTM”), and like plasticizers for vinyl chloride have been developed as non-phthalate ester plasticizers for vinyl chloride-based resin (Patent Documents 1 to 3). Among these plasticizers, ATBC and DOA have the drawback of significantly insufficient heat resistance compared with phthalate ester plasticizers.
In contrast, trimellitate ester plasticizers such as TOTM or tri-n-octyl trimellitate (hereinafter referred to as “n-TOTM”) have heat resistance equivalent to or greater than phthalate esters; therefore, trimellitate ester plasticizers are expected to be plasticizers that have sufficient heat resistance and can be a replacement for phthalate esters.
In particular, among trimellitate esters, the balance between flexibility, cold resistance, heat resistance, and volatility resistance is relatively desirable in TOTM and n-TOTM; therefore, TOTM and n-TOTM are known as plasticizers that have sufficient heat resistance and can be a replacement for phthalate esters.
It is generally known that trimellitate esters currently on the market contain a phthalate ester derived from a raw material. In particular, TOTM and n-TOTM currently used are known to contain a phthalate ester (DOP, di-n-octyl phthalate (hereinafter referred to as “n-DOP”), etc.) (Patent Document 4).
Therefore, a method of reducing the amount of phthalic acid compound contained in the trimellitic acid, which is used as a raw material, has been reported; however, realistically, it is extremely difficult to completely remove a phthalic acid compound. Further, in terms of production costs, production of trimellitate esters becomes more unrealistic as the amount of phthalic acid compound is reduced.
Another attempt that has been known is use of trimellitate triesters obtained from alcohol having 9 or more carbon atoms, i.e., alcohol with an alkyl length with smaller risk and smaller volatile property, such as triisononyl trimellitate (hereinafter referred to as “TINTM”) or triisodecyl trimellitate (hereinafter referred to as “TIDTM”), as plasticizers.
However, since TINTM and TIDTM have drawbacks in terms of flexibility or the like, at present, TINTM and TIDTM cannot be used as TOTM or n-TOTM can be.
Under such circumstances, in view of reducing risk, there has been demand for development of a plasticizer for vinyl chloride-based resin that contains a smaller amount of an ester resulting from a reaction of a phthalic acid compound or derivative thereof and alkyl alcohol having 8 or fewer carbon atoms, and that has superior heat resistance, superior cold resistance, and desirable flexibility.
The “phthalic acid compound” in the present invention is a general name of benzene dicarboxylic acids, including not only orthophthalic acid (narrowly defined) but also isomers such as isophthalic acid, terephthalic acid, and the like.
Further, in recent years, as a non-phthalate ester plasticizer for vinyl chloride-based resins, alicyclic dicarboxylate diesters such as 1,2-cyclohexane dicarboxylate diisononyl (hereinafter referred to as “DINCH”) have been attracting attention as a balanced non-phthalic acid plasticizer having flexibility, heat resistance, and cold resistance similar to those of a phthalate ester plasticizer (Patent Document 5).
However, demand for cold resistance and heat resistance has been further increasing in the use for wire sheathing, automobile parts, and the like. Thus, the demand for heat resistance can no longer be satisfied by DINCH and the like, and the demand for cold resistance can no longer be satisfied by TOTM, n-TOTM, and the like in some cases; therefore, phthalate ester plasticizers are still necessary in some applications. Therefore, a plasticizer as a replacement for phthalate ester plasticizers is strongly desired.
Under such circumstances, in view of reducing risk, there has been a demand for a non-phthalate ester plasticizer for vinyl chloride-based resin that contains a small amount of an ester of a phthalic acid compound and alkyl alcohol having 8 or fewer carbon atoms, and that has superior heat resistance, superior cold resistance, and desirable flexibility.
Previously, the present inventors reported that trimellitate triester obtained by an esterification reaction of trimellitic acid or an anhydride thereof and a specific saturated aliphatic alcohol is useful as a plasticizer for vinyl chloride-based resin having significantly superior volatility resistance and relatively desirable cold resistance and flexibility (Patent Document 6). However, for automobile parts such as interior materials used under severe conditions such as extremely hot weather, a plasticizer with weather resistance and heat discoloration resistance even greater than those of the trimellitate triester mentioned above is desired.
To this end, a method of improving weather resistance, heat discoloration resistance, and the like of a plasticizer by incorporating an antioxidant to a plasticizer has been known. In fact, antioxidants such as 4,4′-(propane-2,2-diyl)diphenol (hereinafter referred to as “bisphenol A”), dibutylhydroxytoluene (hereinafter referred to as “BHT”) and the like have been used.
However, such an improvement cannot be considered fully sufficient to meet the increasing demand for weather resistance and heat discoloration resistance.
Moreover, a common technique for a vinyl chloride-based resin plasticizer has been to make a paste sol by incorporating other components such as a filler together with a vinyl chloride-based resin. A target molded article of a paste sol may be obtained through a paste-making method in which molding such as coating molding, spray molding, dip molding, rotational molding, slush molding, spread molding, calender molding, extrusion molding, press molding, injection molding, or foam molding is performed, and then the molded article is heated and fused. Such a paste-making method is widely used because it enables easy processing.
The properties required for a paste sol are roughly classified into two types of performance, i.e., performance as a paste sol and performance as a molded article that is obtained by processing the paste sol. More specifically, one of the required types of performance is superior processability, i.e., appropriate viscosity characteristics (low viscosity) and storage stability as a paste sol, namely superior sedimentation properties and superior viscosity stability. For the second type of performance, various characteristics such as heat resistance, cold resistance, and flexibility are desired.
More specifically, a plasticizer for a paste sol largely influences the performance of a paste sol, including its flow properties, viscosity stability, and the like, as well as the basic characteristics of the resulting molded article, such as heat resistance, cold resistance, or flexibility. Thus, the selection of the plasticizer is very important.
In the past, phthalate ester plasticizers for vinyl chloride, such as DOP, DINP, or the like, have commonly been used as plasticizers for paste sol. Further, in addition to these phthalate ester plasticizers, for example, TOTM (trimellitate ester plasticizers) and the like have also been used in applications in which heat resistance is required (Patent Document 7). Further, in view of recent environmental issues, ATBC, diisononyl adipate (hereinafter referred to as “DINA”), DINCH and like non-phthalate ester plasticizers are attracting attention (Patent Documents 1, 2, and 8 to 11). In particular, alicyclic dicarboxylate diester plasticizers such as DINCH are attracting attention also in view of their sol characteristics, including viscosity.
As described above, both satisfactory performance as a paste sol and satisfactory performance as a molded article that is obtained by processing the paste sol are required for plasticizers for paste sol. However, no plasticizers for paste sol satisfying both types of performance have been obtained so far.
For example, demand for heat resistance and cold resistance with regard to the molded vinyl-chloride resin articles used for automobile parts and the like are becoming stricter in recent years. Further, for molded vinyl-chloride resin articles to be used for interior materials such as wallpaper, floor materials, or the like, demand for low-VOC (volatile organic compound) products (which cause less indoor volatilization of organic compounds such as plasticizers) has also been increasing.
Examples of plasticizers for satisfying the requirement of low VOC include polyester plasticizers. However, polyester plasticizers have high viscosity, and thus need to be used with adipate ester plasticizers such as DOA; therefore, there have been difficulties for ensuring satisfactory volatility resistance.
Further, there are reports that cyclohexene-1,2-dicarboxylate di-2-ethylhexyl, cyclohexene-1,2-dicarboxylate diisononyl ester, and like alicyclic dicarboxylate diesters can be used as plasticizers for paste sol having low viscosity and superior sol characteristics (Patent Document 12).
However, since none of the molded articles obtained by these alicyclic dicarboxylate diesters fully satisfy the above requirements, further improvement is desired.
Further, as a method for improving sol characteristics, i.e., as a method for reducing viscosity, a method for incorporating a viscosity reducer such as a hydrocarbon compound such as alkyl benzene, mineral spirit, or paraffin; an anionic surfactant; polyoxyethylene alkylphenol ether; sorbitan fatty acid ester; or glycerin fatty acid ester has been widely used.
However, since many of these viscosity reducers are highly volatile, it is becoming more difficult to use them as low-VOC products (Patent Documents 13 to 16).
Since vinyl chloride-based resin compositions are desirable in processability and superior in chemical resistance and durability, molded articles obtained by molding vinyl chloride-based resin compositions have widely been used as medical materials.
Further, the hardness of a vinyl chloride-based resin composition may be varied by changing the proportion of the plasticizer. In particular, since a soft vinyl chloride-based resin composition containing a plasticizer is superior in kinking resistance to polyolefin or the like, it has widely been used as a medical material such as for medical tubes, including catheters, or for medical bags, including blood bags and infusion bags.
Since not only desirable flexibility but also durability, including superior heat resistance to withstand heat treatment and superior cold resistance to withstand storage at low temperature, are required for these soft vinyl chloride-based resin compositions, and since it is also necessary to reduce elution and movability of the additives in view of safety, the selection of a plasticizer having the largest content in a soft vinyl chloride-based resin composition is significantly important.
Until now, phthalate ester plasticizers such as DOP or DINP have commonly been used as the above plasticizer (Patent Document 17). However, since phthalate ester plasticizers have insufficient heat resistance during the heat treatment mentioned above, and also need to be improved in terms of safety, including elution and movability, use of trimellitate ester plasticizers, such as TOTM or polyester plasticizers, have been considered (Patent Documents 18 to 21).
Although trimellitate ester plasticizers and polyester plasticizers are superior in heat resistance, they are inferior in plasticizing efficiency and cold resistance. Thus, in order to obtain sufficient flexibility and cold resistance, it is necessary to incorporate a large amount of a plasticizer to the resin. The incorporation of a large amount of a plasticizer may become problematic in terms of safety or the like. Therefore, at present, there is no balanced plasticizer that can satisfy all of the above characteristics alone.
Under such circumstances, medical materials that ensure improved heat resistance without decreasing flexibility and cold resistance have been desired, and development of a plasticizer and a vinyl chloride-based resin composition enabling production of such materials has been hoped for.
Further, medical materials need to be sterilized and disinfected before use from the standpoint of hygiene. As sterilization and disinfection methods, heat treatments such as dry heat, boiling, or pressurized hot water treatment; application of ultraviolet or radial rays; chemical treatments using ethylene oxide gas or the like are known. Generally, heat treatments or treatments with application of ultraviolet or radial rays are often used.
In recent years, with medical accidents increasing in hospitals, hygiene control in medical practice is becoming even stricter; accordingly, the conditions of the above sterilization and disinfection treatments are also becoming stricter. However, with such increased demand for strict conditions, for example, in heat treatment, a decrease in flexibility due to volatilization of a plasticizer or the like may cause breakage of the material. Thus the required properties may not be satisfied by TOTM or the like. Further, in treatments with application of ultraviolet or radial rays, discoloration causes a decrease in ability to identify the content, thereby causing medical accidents. This is a serious problem. It is known that discoloration may be alleviated by incorporating a large amount of stabilizer (Patent Documents 22 to 25); however, the incorporation of a large amount of stabilizer is significantly problematic in terms of safety, and thus is virtually impossible. Therefore, an effective improvement method has not been found at present.